Nowadays, portable electronic devices are widely used. A battery is attached to the electronic device as a drive power supply. Usage of the electronic device and discharging decreases the output voltage of the battery. Thus, a direct current voltage conversion circuit (DC-DC converter) for converting the voltage of the battery to a constant voltage is arranged in the electronic device. When the load of the DC-DC converter fluctuates, the output voltage must be controlled in accordance with such fluctuation. It is required that the speed for such responses be increased.
In the prior art, a compact switching type DC-DC converter (switching regulator) having satisfactory conversion efficiency is used for a portable electronic device. The switching regulator is a pulse width modulation (PWM) regulator that alternately activates a main switching transistor and a synchronization transistor. Energy is supplied from the input to the output of the switching regulator when the main switching transistor is activated. Energy accumulated in an inductor is discharged when the main switching transistor is inactivated. The output voltage is kept substantially constant by controlling the pulse width of a pulse signal for driving the main switching transistor in accordance with the output voltage or output current.
When the load is low, current flows from the load to ground through the synchronization transistor when the main switching transistor is inactivated (i.e., when the synchronization transistor is activated). This causes energy loss. The following methods have been proposed in the prior art to prevent such energy loss.
Japanese Laid-Open Patent Publication No. 2006-166667 describes a diode rectifying method for connecting a commutation diode (free-wheel) in parallel to the synchronization transistor, and inactivating the synchronization transistor in accordance with the load. Japanese Laid-Open Patent Publication No. 2002-281743 describes a DC-DC converter for detecting a reversed flow of current and inactivating the synchronization transistor. Japanese Laid-Open Patent Publication No. 2002-354787 describes a DC-DC converter for estimating the current that flows through a coil and inactivating the synchronization transistor in accordance with the estimated current.